Mark Dunn, Senior National Reporter

OTTAWA - The NDP says the prime minister is covering up for a batch of Conservative senators he selected who are now the target of a secretive Red Chamber investigation into misused public funds.

Stephen Harper was grilled in the Commons for a third day on a scandal the NDP now says ensnares at least 17 senators - including Liberal Mac Harb - who the opposition says fudged residency rules to pocket a rich housing allowance.

The Senate won't say how many of its members have been hauled before a sub-committee to prove they haven't lied to collect hundreds of thousands of dollars in perks.

A Senate spokeswoman said it would be "inappropriate to speculate" about the number being grilled, whether any have repaid ill-gotten benefits and if a report into the hush-hush probe would be released.

The government says a report will be made public.

NDP Leader Thomas Mulcair said he has no faith taxpayers will learn how some of the more than $100 million allotted annually on the Senate is spent.

"We have an unelected, unaccountable and unapologetic Senate. You're not going to find out what went on," he said.

Harper said "all senators conform to the residency requirements" and accused the NDP of blocking legislation that would elect senators "because they want to appoint their own." "But guess what? The people of Canada are never going to give them that chance."